As examples of a resin material conventionally used in a toner, may be mentioned polystyrene resins, styrene-acrylic copolymer resins, polyester resins, epoxy resins, butyral resins, and hybrid resins such as polyester resins having grafted acrylic resins. The design of a resin material depends on an application of the toner.
Especially, in a resin material of the toner for heat roller fixing, fixing properties to a recording medium and improvement in offset resistance are required. Thus, a thermoplastic resin having a high molecular weight or a partially crosslinked thermoplastic resin has been largely employed.
As printers and copying machines are operated at higher speed with further saved energy, a toner having excellent low temperature fixability is increasingly required. When the resin material as described above is employed, the temperature for melting and fixing a toner (fixing temperature) needs to be set high. Therefore, it is difficult to achieve energy saving.
For developing low temperature fixability in a toner, a resin material having a low melting temperature and a low melting viscosity needs to be employed. For this purpose, it is important to use a resin material having a low glass transition temperature (Tg) and a low molecular weight.
However, there arises a new problem in that the toner including such a resin material has a low heat-resistant storage properties (blocking resistance).
Thus, it is essentially difficult to balance between low temperature fixability and heat-resistant storage properties in a toner.
For solving the above-described problem, there has been proposed a toner in which a non-crystalline resin is contained in a core particle and the surface thereof is covered with a crystalline polyester resin (for example, see Patent Literature 1).
However, a crystalline polyester resin has a property of being hard but brittle. Therefore, the toner is easily crushed when stirred in a developing device. The crush of a toner significantly occurs especially in a high-speed machine.
Patent Literature 2 has proposed a technology of mixing a crystalline resin having a low melting point and a non-crystalline resin, and controlling the compatibility to obtain low temperature fixability.
However, as the compatibility between a crystalline resin and a non-crystalline resin proceeds, plasticization of the resin mixture occurs. Therefore, there is a problem in that sufficient heat-resistant storage properties (blocking resistance) cannot be obtained.
In a vinyl-based resin such as a styrene-acrylic copolymer resin having high versatility, a resin having a low molecular weight needs to be employed in order to develop low temperature fixability. However, in this case, there is a problem in that sufficient crush resistance cannot be obtained.
When such a toner is used for an extended period, the toner is fractured when subjected to a friction with a carrier in a developing device to become a fine toner. The fine toner is likely to adhere to the surface of a carrier. The fine toner is further fused to a carrier, causing a charge providing function of a carrier to decrease. Therefore, the charge amount of a toner decreases. As a result, the toner with charge defects is scattered. Therefore, there is a problem such as occurrence of background fogging.
In the end, in the toner containing a crystalline resin, low temperature fixability, which is an advantage of a crystalline resin, can be obtained, but heat-resistant storage properties and crush resistance cannot be sufficiently satisfied.